In a typical computer system, numerous signal lines are used to connect integrated circuits. Information is transmitted between integrated circuits over these signal lines through different signal levels, with the ability to switch from one signal level to another often being the factor limiting the maximum rate of information transmission. The ability to switch a signal line is not only affected by the characteristics of the driver that is causing the transition, but also by the characteristics of the signal line, the other devices attached to the signal line, and the termination, if any, of the signal line.
At least one integrated circuit coupled to a signal line typically has an output buffer for driving the signal line. As technology progresses, these output buffers are able to drive signals having faster transitions. The higher frequency components inherent in the faster transitions are well known to increase various transmission line effects including ringing and wave reflection. Thus, increasing measures are required to compensate for transmission line effects.
Many prior art signaling schemes utilize "open" termination in which signal lines are terminated in a high impedance input buffer or a tristated output buffer. Many of these schemes include diodes for electrostatic discharge (ESD) protection. FIG. 1 shows a prior art circuit in which an output buffer 122 drives data from a data line 124 when enabled using an enable line 126. In this prior art signaling scheme, a signal line is switched between a Vcc potential and a Vss potential. When output buffer 122 is tristated, an ESD diode 120 clamps voltage swings on an interface node 125 of more than one diode drop over the Vcc potential to which the diode is coupled. ESD diode 130 clamps voltage swings of more than one diode drop under the Vss potential to which it is coupled.
Voltage wave reflections result when a voltage wave reaches an impedance mismatch at the interface between a signal line and a high impedance termination. ESD diodes provide some reflection limitation; however, the remaining reflections may still result in system malfunction due to the overdriving of other devices. Also, an initial reflection from a high impedance end of the signal line may be again reflected by the driver, resulting in multiple reflections with the possibility of causing receivers to sample improper signal levels. Finally, as switching frequencies and edge rates increase, reflections and other transmission line effects in a system with "open" ended signal lines tend to worsen.
Due to the problems posed by reflections on signal lines with "open" terminations, a variety of signal line terminations are used. One common signaling scheme terminates a signal line to a fixed potential using a termination resistor. The signal line may be terminated to Vcc, ground, or an intermediate potential through the termination resistor. Using a termination reduces incident wave reflection, typically allowing receivers to properly sample the signal line upon receiving the incident wave (i.e. incident wave switching). Assuming the termination resistor is connected to Vcc, a high logic level appears on the signal line as Vcc. A low logic level appears on the signal line as a voltage with a magnitude greater than Vss determined according to the ratio of signal driver impedance to the total of the driver and the termination impedances. While the voltage swing is reduced as compared to the "open" termination case, the reduction in reflections and the ability to utilize incident wave switching make resistively terminated signal lines a solution for some applications.
While resistively terminated signal lines may reduce reflections, they can have the disadvantage of increasing power consumption. In a system with a resistive termination to Vcc, an output buffer driving a signal line to ground draws continuous current through the resistor. Considering the increasing importance of power savings in computer systems, the increased power consumption of a typical resistive termination scheme is a significant disadvantage.
One prior art scheme which may reduce some of the problems with resistively terminated signal lines is addressed in "A Dynamic Line-Termination Circuit for Multireceiver Nets", IEEE Journal on Solid-State Circuits, Vol. 28, No. 12, December 1993. Here the receiver terminates a line to incoming signal values. This scheme essentially implements a keeper formed by an input buffer and an active termination device which immediately drives values back onto the line. The keeper thus latches values as they are received on the signal line. Since the termination device terminates the signal line to the signal value being driven, there is not continuous additional power consumption due to the termination; however, the switching of the active termination device during an incident voltage wave can produce a step voltage on the signal line.
In a typical application, many such dynamic termination circuits are used on a signal line, and each termination circuit will contribute to signal line noise with its step voltage, resulting in the incident voltage wave being altered by each of the termination circuits. These noise causing step voltages occur at different times since the receivers are generally located at different distances from the output buffer. Additionally, once all of the receivers have switched and are terminating the signal line to its present value, the next time a driver tries to drive the opposite value on the signal line, it will have to overcome all of the active termination devices. This may result in poor performance in a system with many receivers. Thus the dynamic line termination scheme addresses the power consumption disadvantage of resistively terminated signal lines, but has some shortcomings of its own.